Folding apparatuses are used in conjunction with rotary printing presses in order to fold webs (or ribbons) of printed material. Generally, a full width web of material is cut longitudinally into a number of ribbons by a slitter mechanism prior to folding in the folder apparatus. However, for purposes of this application, the term web encompasses the term ribbon as well.
A folding apparatus is generally coupled to one or more rotary printing presses. Each folding apparatus generally includes a superstructure through which two or more webs of flat printed material are passed before they reach a cylinder unit of the folding apparatus. The superstructure includes a take-off roller corresponding to each web for driving the webs into the cylinders of the folding apparatus. A problem which frequently arises is that the circumferential velocities at which the various take-off rollers must be driven are often different from one another.
Varying circumferential velocities are necessary in order to achieve an identical transport speed of the various webs. For example, it may be necessary to ensure that a predetermined position on the imprint of one web arrives at a cutting device at the same time that the imprint of another web arrives at that cutting device. The necessity for different circumferential velocities of the take-off rollers results, for example, from the different web lengths within a turning rod section of the folder or between a turning rod and a related take-off roller, and from the different tension conditions of the various webs which result from this.
The necessity for different circumferential velocities also results from the so-called radius effect, which occurs if several webs are passed over a single roller, i.e. one above the other. In this case, the webs which are passed over the outside of the roller (e.g. the outermost web) are pulled off more rapidly than the ones which pass over the inside of the roller (i.e. the web which directly contacts the roller), because of their greater distance from the center of the roller. If the webs which are passed further to the outside are now passed to the inside on a subsequent roller, in other words at a slower web velocity in comparison with the preceding roller, wrinkles and other undesirable irregularities of the web product can occur. In order to compensate for such radius effects, the circumferential speed of the subsequent roller is increased by utilizing complicated control mechanisms.
In addition to the technical effort of providing a separate control for each drive roller of a large drive roller assembly, adjustment and monitoring of the circumferential velocity of a large number of rollers can easily overburden the operating personnel. While it is possible to use step-down gears, or separate drives which allow precise velocity adjustment, these devices are cost-intensive and require careful installation. Such devices can also reduce the operational reliability of the system because they have a tendency to break-down.